Title:
The linear polarisation of southern bright stars measured at the parts-per-million level

Abstract: We report observations of the linear polarisation of a sample of 50 nearby
southern bright stars measured to a median sensitivity of $\sim$4.4 $\times
10^{-6}$. We find larger polarisations and more highly polarised stars than in
the previous PlanetPol survey of northern bright stars. This is attributed to a
dustier interstellar medium in the mid-plane of the Galaxy, together with a
population containing more B-type stars leading to more intrinsically polarised
stars, as well as using a wavelength more sensitive to intrinsic polarisation
in late-type giants. Significant polarisation had been identified for only six
stars in the survey group previously, whereas we are now able to deduce
intrinsic polarigenic mechanisms for more than twenty.
The four most highly polarised stars in the sample are the four classical Be
stars ($\alpha$ Eri, $\alpha$ Col, $\eta$ Cen and $\alpha$ Ara). For the three
of these objects resolved by interferometry, the position angles are consistent
with the orientation of the circumstellar disc determined. We find significant
intrinsic polarisation in most B stars in the sample; amongst these are a
number of close binaries and an unusual binary debris disk system. However
these circumstances do not account for the high polarisations of all the B
stars in the sample and other polarigenic mechanisms are explored. Intrinsic
polarisation is also apparent in several late type giants which can be
attributed to either close, hot circumstellar dust or bright spots in the
photosphere of these stars. Aside from a handful of notable debris disk
systems, the majority of A to K type stars show polarisation levels consistent
with interstellar polarisation.

Comments:

22 pages, 10 figures, 12 tables. Accepted to MNRAS 2015 September 18. Received 2015 September 18; in original form 2015 August 19